On Sunday the couple posted an Instagram picture of themselves preparing to sign official documents. "That's the legal bit done. Now on to the ceremony!" they wrote.
John and Furnish planned a celebration at their estate near Windsor Castle west of London. An A-list crowd is expected -- even if the festivities are likely to be much more sedate than the parties in John's hell-raising rock 'n' roll days.
The singer -- known for an impressive string of hits dating back decades -- promised to post pictures of the festivities to fans around the world.
The nuptials came as no surprise: John had tweeted about his plans earlier in the week, and earlier in the year had praised England for embracing same-sex marriage. He said there had been a remarkable change in a country where homosexual acts had long been deemed criminal offenses.
John said the wedding would be a joyous occasion shared with the couple's two sons, Zachary and Elijah.
The singer had said earlier this year that he and Furnish planned to take advantage of the changes to the law, even though they already had a civil bond recognized by the courts.
"We shouldn't just say, 'Oh well, we have a civil partnership, we're not going to bother to get married," he said.
John and Furnish met in 1993. Both have been active in a number of charities, with John focusing on efforts to combat the spread of HIV and AIDS.
He was knighted by Queen Elizabeth II in 1998 in recognition of his charitable works.